Identification of fertility restorer lines in rice for WA-male sterile cytoplasm

Rice is the staple food consumed by almost half of the population in Asia and the world. Rice is a self-fertile plant with various varieties. Due to the decrease in the area where rice is cultivated, there is a great need to increase yield performance in rice per unit area. Heterosis is a phenomenon that reveals advantages over its parents in F1 hybrids resulting from the crossbreeding of various parents (Saleem, 2008). The two main methods for producing hybrid rice are the three-line system and the two-line system. These two methods are different due to the use of two different types of male sterile lines. In the production of F1 hybrids, CMS lines are crossed with fertility restorer lines. The objectives of this study were to evaluate the new fertility restorer lines in terms of important crop characteristics and determine the fertility percentage of pollen grains and evaluate the heterosis rate of the resulting hybrids.

The research materials used in this study included 16 new restorer lines as paternal parents and NedaA cytoplasmic male sterility lines as maternal parent as well as its fertility maintainer lines NedaB. In the first year, the agronomic characteristics including plant height (cm), number of fertile tiller, panicle length (cm), number of grains, number of filled grains, grain length (millimeters), grain diameter (millimeters), weight of one thousand grains (grams) and grain yield (grams) were recorded and at the flowering stage, all fertility restorer lines were crossed with NedaA CMS line. Among the available hybrids, 7 F1 genotypes were cultivated with their parents in the second year. After flowering, iodine-potassium iodide (I₂/KI) solution was used to perform fertility assessment of pollen grains in F1 hybrids (Gao et al, 2011). At maturity stage seed setting performance of F1 genotypes recorded. Analysis of variance and mean comparisons using Duncan test were made using SPSS software. Grouping of genotypes was made using cluster analysis through Ward method.

Mean comparison of genotypes showed that K14, K3, K4 and K11 lines had the highest average height, respectively, which was a positive feature for paternal parents (pollinators) as restorer genotypes. In terms of number of fertile tiller, K4, K7 and K1 restorers accounted for the largest number of fertile tillers, respectively, and K4 genotype was superior to other genotypes in terms of panicle length. Researchers identified the fertile tillers, panicle length, and number of filled grains as the most contributing traits to yield. Lines K16, K12 and K4 had the highest average grain length per panicle. This feature is one of the important traits of marketing in Iran. Lines K12, K4, K3 and K2 had the highest weight of one thousand grains, respectively. Cluster analysis classified the genotypes into two main categories (Figure 1). The genotypes in same group were more genetically related than the genotypes in other branches, hence, cluster analysis can be used as a tool to select parental lines in hybridization programs (Kiani &Nematzadeh, 2013). The first group included 11 genotypes (68.75%) that were below average in terms of all traits except grain diameter. 5 genotypes (31.25%) were placed in the second cluster. The K12 and K16 genotypes were included in this group as two successful parents in cross with the NedaA male sterile genome. The fertility rate of hybrid pollen grains ranged from 41.71 to 95.35 percent. NedaA/K7, NedaA/K12 and NedaA/K16 hybrids had more than 80% fertility rate, which indicated the suitability of K7, K12 and K16 restorer lines for crossing with Neda A.All the promising hybrids outperformed their paternal parents in terms of grain length. The highest 1000 grain weight was observed in NedaA/K16 hybrid and the highest yield was observed in NedaA/K7. Heterobeltiosis of the NedaA/K7 and NedaA/K12 genotypes were 93.47% and 98.95%, respectively and the NedaA/K16 genotype showed 26.22% of Heterosis.

Three genotypes (K7, K12 and K16) were found to be desirable fertility restorer lines for NedaA due to their beneficial characteristics as well as representing more than 80 percent fertility percentage of pollen grains and seed setting in their panicles. These lines could be used as new sources of fertility restoration for WA cytoplasmic sterility system and promote hybrid seed technology in Iran.